ecosystem -- All the organisms in a particular region and the environment in which they live. The elements of an ecosystem interact with each other in some way, and so depend on each other either directly or indirectly.

ectoderm -- The outer basic layer of tissue in those animals with true tissues. In vertebrates, for instance, the embryonic ectoderm differentiates into the skin and also the nervous system.

egg -- (1) A large gamete without flagellae that is fertilized by a sperm cell. An egg cell is also called an ovum. (2) A complex multicellular structure in which an animal embryo develops.

elater -- A cell or part of a cell which assists in dispersing spores. The elaters change shape as they lose or acquire water, and they will then push against surrounding spores.

embryo -- The earliest stage of development in an animal or plant. An embryo begins to form following the fusion of egg and sperm (a zygote).

embryophyte -- Synonym for the Plantae, as here defined. It includes all green photosynthetic organisms which begin the development of the sporophyte generation within the archegonium.

enations -- Flaps of tissue such as those found on psilophytes.

endoderm -- The innermost basic layer of tissue in those animals with true tissues. Forms the gut and its derivatives: in vertebrates, these include the liver, trachea, and lungs.

endodermis -- Literally "inner skin", this is a layer of cells which surrounds the central core of vascular tissue, and which helps to regulate the flow of water and dissolved substances.

endoplasmic reticulum -- (ER) network of membranes in eukaryotic cells which helps in control of protein synthesis and cellular organization.

endosymbiosis -- When one organism takes up permanent residence within another, such that the two become a single functional organism. Mitochondria and plastids are believed to have resulted from endosymbiosis.

entomophily -- Seed plants which are pollinated by insects are said to be entomophilous.

environment -- The place in which an organism lives, and the circumstances under which it lives. Environment includes measures like moisture and temperature, as much as it refers to the actual physical place where an organism is found.

enzyme -- complex protein which helps to speed biochemical reactions. Enzymes are important in the construction and degradation of other molecules.

epicenter -- Point on the Earth's surface directly above the focus of an earthquake.

epidermis -- The outermost layer of cells or skin. This tissue often contains specialized
cells for defense, gas exchange, or secretion.

epiphyte -- A plant which grows upon another plant. The epiphyte does not "eat" the plant on which it grows, but merely uses the plant for structural support, or as a way to get off the ground and into the canopy environment.

epithelium -- Layer of cells which lines a body cavity; cells may be ciliated or unciliated, and may be squamous (flat, scale-shaped), cuboidal (cube-shaped), or columnar (column-shaped). Your stomach and cheeks are lined with epithelium.

erosion -- n. The processes by which materials of the Earth's crust are worn away, loosened, or dissolved while being transported from their place of origin.

esker -- n. A ridge of glacial sediment deposited by a stream flowing in and under a melting glacier.

esophagus -- That portion of the gut which connects the pharynx to the stomach.

estuary -- n. An area where fresh water comes into contact with seawater, usually in a partly enclosed coastal body of water; a mix of fresh and salt water where the current of a stream meets the tides; estuarine– adj.

eukaryote -- n. An organism whose cells have cytoskeletons for support and their DNA contained in a nucleus, separated from the other contents of the cell; e.g., protists, plants, animals, and fungi; eukaryotic- adj.

Euramerica -- n. A supercontinent that existed in the Late Silurian through Devonian, formed by the collision of Baltica, Laurentia, and Avalonia; included modern North America, Greenland, Scandinavia, and Europe; also called the “Old Red Continent” for the red color of its oxidized deposits.

eustele -- When a plant's vascular tissue develops in discrete bundles, it is said to have a eustele. See also protostele and siphonostele.

evaporite -- n. a deposit of salt minerals (e.g., halite, gypsum, anhydrite) left behind by the evaporation of seawater, usually forms within a restricted basin.

evolution -- Darwin's definition: descent with modification. The term has been variously used and abused since Darwin to include everything from the origin of man to the origin of life.

evolutionary tree -- A diagram which depicts the hypothetical phylogeny of the taxa under consideration. The points at which lineages split represent ancestor taxa to the descendant taxa appearing at the terminal points of the cladogram.

exoskeleton -- n. An external, often hard, covering or integument that provides support and protection to the body.

extinction -- When all the members of a clade or taxon die, the group is said to be extinct.

extracellular matrix -- (ECM) Region outside of metazoan cells which includes compounds attached to the plasma membrane, as well as dissolved substances attracted to the surface charge of the cells. The ECM functions both to keep animal cells adhered together, and well as buffering them from their environment.